1. Field of the Invention
The present invention relates to an electrostatic chuck device, and more particularly, to an electrostatic chuck device suitable for use in a high-frequency discharge type plasma processing apparatus for applying a high-frequency voltage to an electrode to generate plasma and processing a plate-like sample such as a semiconductor wafer, a metal wafer, and a glass plate by the use of the generated plasma.
2. Description of the Related Art
Conventionally, plasma was often used in processes such as etching, deposition, oxidation, and sputtering for manufacturing semiconductor devices such as IC, LSI, and VLSI or flat panel displays (FPD) such as a liquid crystal display, in order to allow a process gas to react sufficiently at a relatively low temperature. In general, methods of generating plasma in plasma processing apparatuses are roughly classified into a method using glow discharge or high-frequency discharge and a method using microwaves.
FIG. 7 is a sectional view illustrating an example of an electrostatic chuck device 1 mounted on a known high-frequency discharge type plasma processing apparatus. The electrostatic chuck device 1 is disposed in a lower portion of a chamber (not shown) also serving as a vacuum vessel and includes an electrostatic chuck section 2 and a metal base section 3 fixed to the bottom surface of the electrostatic chuck section 2 so as to be incorporated into a body.
The electrostatic chuck section 2 includes: a substrate 4, which has a top surface serving as a mounting surface 4a, on which a plate-like sample W such as a semiconductor wafer is disposed, so as to adsorb the plate-like sample W in an electrostatic manner, and an electrostatic-adsorption inner electrode 5 built therein; and a power supply terminal 6 for applying a DC voltage to the electrostatic-adsorption inner electrode 5. A high DC voltage source 7 is connected to the power supply terminal 6. The metal base section 3, which is also used as a high-frequency generating electrode (lower electrode), is connected to a high-frequency voltage generating source 8 and has a flow passage 9 for circulating a cooling medium such as water or an organic solvent formed therein. The chamber is grounded.
The electrostatic chuck device 1 adsorbs the plate-like sample W, by placing the plate-like sample W on the mounting surface 4a and allowing the high DC voltage source 7 to apply a DC voltage to the electrostatic-adsorption inner electrode 5 through the power supply terminal 6. Subsequently, a vacuum is generated in the chamber and a process gas is introduced thereto. Then, by allowing the high-frequency voltage generating source 8 to apply high-frequency power across the metal base section 3 (lower electrode) and an upper electrode (not shown), a high-frequency electric field is generated in the chamber. Frequencies of several tens of MHz or less are generally used as the high frequency.
The high-frequency electric field accelerates electrons, plasma is generated due to ionization by collision of the electrons with the process gas, and a variety of processes can be performed by the use of the generated plasma.
In the recent plasma processes, there is an increased need for processes using “low-energy and high-density plasma” having low ion energy and high electron density. In the processes using the low-energy and high-density plasma, the frequency of the high-frequency power for generating plasma might increase greatly, for example, to 100 MHz.
In this way, when the frequency of the power to be applied increases, the electric field strength tends to increase in a region corresponding to the center of the electrostatic chuck section 2, that is, the center of the plate-like sample W, and to decrease in the peripheral region thereof. Accordingly, when the distribution of the electric field strength is not even, the electron density of the generated plasma is not even and thus the processing rate varies depending on in-plane positions in the plate-like sample W. Therefore, there is a problem in that it is not possible to obtain a processing result excellent in in-plane uniformity.
A plasma processing apparatus shown in FIG. 8 has been suggested to solve such a problem (see Patent Document 1).
In the plasma processing apparatus 11, in order to improve the in-plane uniformity of the plasma process, a dielectric layer 14 made of ceramics or the like is buried at the central portion on the surface of the lower electrode (metal base section) 12 supplied with the high-frequency power and opposed to the upper electrode 13, thereby making the distribution of the electric field strength even. In the figure, reference numeral 15 denotes a high frequency generating power source, PZ denotes plasma, E denotes electric field strength, and W denotes the plate-like sample.
In the plasma processing apparatus 11, when the high frequency generating power source 15 applies the high-frequency power to the lower electrode 12, high-frequency current having been transmitted on the surface of the lower electrode 12 and having reached the top due to a skin effect flows toward the center along the surface of the plate-like sample W, and a part thereof leaks toward the lower electrode 12 and then flows outward inside the lower electrode 12. In this course, the high-frequency current is submerged deeper in the region provided with the dielectric layer 14 than the region not provided with the dielectric layer 14, thereby generating hollow cylindrical resonance of a TM mode. As a result, the electric field strength of the central portion supplied to the plasma from the surface of the plate-like sample W is weakened and thus the in-plane electric field of the plate-like sample W is made to be uniform.
The plasma process is often performed under depressurized conditions close to a vacuum. In this case, an electrostatic chuck device shown in FIG. 9 is often used to fix the plate-like sample W.
The electrostatic chuck device 16 has a structure such that a conductive electrostatic-adsorption inner electrode 18 is built in a dielectric layer 17. For example, the conductive electrostatic inner electrode is interposed between two dielectric layers formed by thermally spraying alumina or the like.
The electrostatic chuck device 16 adsorbs and fixes the plate-like sample W by the use of the electrostatic adsorption force generated on the surface of the dielectric layer 17 by allowing the high DC voltage source 7 to apply the high DC power to the electrostatic-adsorption inner electrode 18.
[Patent Document 1] Japanese Patent Unexamined Publication No. 2004-363552 (see paragraphs 0084 and 0085 of page 15 and FIG. 19)
However, even in such an electrostatic chuck device, because the potential of the plasma above the center portion of the plate-like sample W becomes high and the potential at the peripheral portion thereof becomes low, the processing rate differs at the center portion and the peripheral portion of the plate-like sample W, and there is a problem in that this is a factor causing in-plane unevenness in a plasma process such as etching. In addition, the action and responsiveness of the electrostatic adsorption force were also inadequate.
Thus, as a result of intensive investigations to solve the shortcomings described above, the inventors found that the volumetric resistance of the electrostatic-adsorption inner electrode of the electrostatic clutch device must be set within a range of 1.0×10−1 Ωcm to 1.0×105 Ωcm, and preferably, from 1.0×102 Ωcm to 1.0×104 Ωcm.
In addition, the following types of sintered bodies may be provided as examples of materials that have such a volumetric resistance:
(1) a sintered body in which a high melting point metal such as molybdenum (Mo), tungsten (W), and tantalum (Ta) is added to insulating ceramics such as alumina (Al2O3);
(2) a sintered body in which conductive ceramics such as tantalum nitride (TaN), tantalum carbide (TaC), and molybdenum carbide (Mo2C) are added to an insulating ceramic such as alumina (Al2O3); and
(3) a sintered body in which a conductor such as carbon (C) is added to an insulating ceramic such as alumina (Al2O3).
However, when fabricating the electrostatic-adsorption inner electrode by using the sintered bodies in (1) to (3) described above, it is difficult to evenly mix conducting components such as high melting temperature metals, conducting ceramics, and carbon, with insulating ceramics on an industrial scale. Thus, the proportions of these conducting components easily deviate from the proportions that are necessary for obtaining the target volumetric resistance value. Accordingly, when these conducting components vary even slightly, the volumetric resistance value varies significantly. Thus, this volumetric resistance does not attain a desired constant value, and therefore there is a problem in that the volumetric resistance easily deviates from a range of 1.0×10−1 Ωcm to 1.0×105 Ωcm, and preferably, from 1.0×102 Ωcm to 1.0×104 Ωcm, and preparing the volumetric resistance of the electrostatic-adsorption inner electrode so as to attain the desired constant value is extremely difficult.
In addition, in an industrial scale heat treatment furnace that is used when fabricating an electrostatic chuck device, the temperature distribution inside the furnace is not even, and normally there is a variation in the temperature of about ±25° C. to ±50° C. Thus, when fabricating this electrostatic chuck device, a conductive material layer, which forms the electrostatic-adsorption inner electrode and includes the raw components of the above-described sintered bodies (1) to (3), is interposed between a mounting plate on which the plate-like sample is mounted and a supporting plate that supports this mounting plate. Subsequently, when these are baked and the mounting plate, electrostatic-adsorption inner electrode, and the supporting plate are integrated by bonding to form an integrated body, the volumetric resistance of the electrostatic-adsorption inner electrode that has been produced is significantly influenced by the temperature distribution in the furnace. Thus, this volumetric resistance does not attain the desired constant value, and therefore there is a problem in that the volumetric resistance easily deviates from the range of 1.0×10−1 Ωcm to 1.0×105 Ωcm, and preferably, from 1.0×102 Ωcm to 1.0×104 Ωcm, and preparing the volumetric resistance of the electrostatic-adsorption inner electrode so as to attain a desired constant value becomes extremely difficult.
Accordingly, the volumetric resistance of the electrostatic-absorption inner electrode is easily influenced by the variations in the conductive component and variations in the temperature during baking, and thus stably obtaining the desired constant value is difficult. Consequently, it is difficult to realize a uniform plasma process on the plate-like sample and to obtain an electrostatic chuck device in which the action and responsiveness of the electrostatic adsorption force are advantageous.
In consideration of the circumstances described above, it is an object of the invention to provide an electrostatic chuck device in which, when applied to a plasma process apparatus, the in-plane uniformity of the electric field strength in the plasma is improved, and a plasma process having a high in-plane uniformity with respect to the plate-like sample can be carried out.